MAINTAINERS: give proc sysctl some maintainer love

We poke at proc sysctl enough that really we should declare it
maintained.  We'll just be Cc'd and sending updates / ACK'ing changes
through akpm's tree.

Link: http://lkml.kernel.org/r/20170524231305.8649-1-mcgrof@kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>